1. Field of the Invention
This invention relates to a method and apparatus for coating a loop with viscous material, and particularly to a method for coating a seal edge of a funnel portion of a television cathode ray tube with frit slurry in order to fix the seal edge of the funnel portion to an end surface of a panel portion of the television cathode ray tube.
2. Description of the Prior Art
As shown in FIG. 1, when a picture tube or a television cathode ray tube is manufactured, a funnel portion 1 and a nearly rectangular bowl-shaped panel portion 2 are prepared, and they are attached to each other at their end surfaces. In the attaching operation, an end surface or seal edge 1a of the funnel portion 1 is coated with a so called frit slurry. For example, a typical frit slurry composition contains 10 kg frit (Asahi 1307-Tradename-manufactured by Asahi Glass Co., Ltd.) which consists of 70% PbO, 13% ZnO and the remainder SiO.sub.2 and B.sub.2 O.sub.3, and 900 to 1000 cc of a carried therefor consisting of isoamyl acetate solvent and 1.1 to 1.3% by weight, based on weight of the carrier of nitrocellulose wetted with isopropyl alcohol as a binder.
The FIG. 2 shows a prespective view of a known apparatus for coating the seal edge la of the funnel portion 1 with frit slurry and the principal of operation of the apparatus. In the FIG. 2 apparatus, the funnel portion 1 is rotated around its central axis in a counterclockwise direction by a supporting member (not shown). A nozzle body 3 containing frit slurry is arranged so as to be urged towards the central axis of the funnel portion 1. A follower portion (not shown) is formed integrally with the nozzle body 3 and is pressed against and follows an outside marginal portion of the funnel portion 1. Accordingly, the nozzle body 3 is guided along the seal edge 1a of the funnel portion 1 with the rotation of the funnel portion 1. Frit slurry is discharged towards the seal edge 1a from the outlet opening of the nozzle body 3 at a constant rate, so that the whole seal edge 1a of the funnel portion 1 is coated with frit slurry. In such a method and apparatus, the coating operation starts on an initial point of the seal edge 1a, and ends on a terminal point of the seal edge 1a after making one circuit of the seal edge 1a. The initial point coincides approximately with the terminal point. During the coating operation, the frit slurry discharged on the seal edge 1a gradually becomes hard due to its thixotropy and vaporization of the solvent of the frit slurry and contracts somewhat as it hardens. Accordingly, a slight double layer of frit slurry is formed on the terminal point of the seal edge 1a so as to satisfactorily combine both ends of the coated layer of frit slurry. When frit slurry is discharged onto the terminal point, that is onto the initial point or slightly beyond the initial point after making one circuit of the seal edge 1a, the layer of frit slurry swells and projects slightly above the terminal or initial point of the seal edge 1a. The thickness of the layer of frit slurry is thus not uniform at the initial or terminal point of the seal edge 1a. Moreover frit slurry somewhat flows out to the outside marginal portion of the funnel portion 1 from the terminal or initial point. Thus, neither the height nor the width of the terminal or initial point are uniform with the remainder of the slurry. Because of the lack of uniformity of the height and/or width of the slurry, the panel portion 2 cannot be exactly fitted to the funnel portion 1. Heretofore, the nonuniform points has been manually corrected with a knife or spatula to cut away the excess frit slurry material. However, such an operation requires considerable skill, and it has been an obstacle to the efficient manufacture of the cathode ray tube.
Moreover, particles of frit slurry are often scattered about in the manual correction operation. The particles are deposited on different parts of the funnel portion 1. There is a possibility that the scattered particles of frit slurry causes deterioration of the insulation or conductivity of different parts of the cathode ray tube so as to shorten the life time of the cathode ray tube, and that the scattered particles of frit slurry have an adverse influence on the picture of the cathode ray tube. Moreover, frit slurry is harmful to the health of factory assembly personnel and a considerable expense is involved in protecting assembly personnel from the particles of frit slurry.